Add FolkmanGraph

[finnbuck]

Added functions for constructing a Folkman Graph. @james-d-mitchell

[codecov]

Codecov Report

Merging #386 (038ddd5) into master (0c909dd) will decrease coverage by 1.66%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
- Coverage   97.01%   95.34%   -1.67%     
==========================================
  Files          45       50       +5     
  Lines       12884    13121     +237     
==========================================
+ Hits        12499    12510      +11     
- Misses        385      611     +226     

Impacted Files	Coverage Δ
gap/examples.gd	100.00% <100.00%> (ø)
gap/examples.gi	98.95% <100.00%> (+0.03%)	⬆️
src/bitarray.c	93.33% <0.00%> (-1.91%)	⬇️
gap/attr.gi	99.31% <0.00%> (-0.69%)	⬇️
src/planar.c	93.75% <0.00%> (-0.37%)	⬇️
src/perms.c	77.77% <0.00%> (-0.35%)	⬇️
src/homos-graphs.c	98.66% <0.00%> (-0.27%)	⬇️
src/homos.c	99.08% <0.00%> (-0.07%)	⬇️
src/cliques.c	99.26% <0.00%> (-0.04%)	⬇️
gap/labels.gi	98.64% <0.00%> (-0.03%)	⬇️
... and 18 more

[finnbuck]

We have decided to go a different direction with named graphs. However, this version of the Folkman Graph implementation could still be useful as a template.

[james-d-mitchell]

Thanks @finnbuck we did decide to go in a different direction for sporadic (i.e. non-infinite families of) digraphs, but I think we should leave this open for the purpose that you mentioned @finnbuck, as an exemplar of what to do to add an infinite family. Thanks for this!

[olexandr-konovalov]

Looks good to me - thanks!

[wilfwilson]

If this is not intended to be merged, I think it's probably an appropriate point to close this PR now, right? (Seeing as there are already several examples of this kind that have now been merged).

[james-d-mitchell]

Closing this as it was an exemplar only!